Enantiomers

Two stereoisomers that are mirror images of each other are almost the same compound. They boil at the same temperature, dissolve in the same solvents, and show identical spectra — every routine instrument in the lab reports them as one and the same compound. The only things that tell them apart are polarized light and other chiral molecules — and the second of those is why the distinction matters enormously in biology and medicine.

The definition

Enantiomers are stereoisomers that are non-superimposable mirror images of each other. It is the chirality relationship from two sections back, now applied to a specific pair of molecules.

A single stereocenter has exactly two possible configurations, so a molecule with one stereocenter has exactly one enantiomer. With more stereocenters, a molecule still has exactly one enantiomer — the one in which every stereocenter is inverted. Invert only some of them and you get a diastereomer instead, which is the subject of the next section.

Identical physical properties, with one exception

Enantiomers have identical melting points, boiling points, densities, refractive indices, solubilities in ordinary solvents, IR spectra, NMR spectra and reactivity toward achiral reagents. Every ordinary measurement gives the same answer for both, and that is not a limitation of the technique — the two molecules genuinely have the same internal energy and the same set of bond distances and angles.

The exception is that they rotate plane-polarized light in opposite directions by equal amounts. This property is optical activity, and historically it is how enantiomers were discovered — Pasteur separated the two crystal forms of a tartrate salt by hand in 1848, long before anyone could describe a molecule in three dimensions.

Specific rotation

The measured rotation depends on how much sample the light passed through, so it is normalized into a reportable constant:

where α is the observed rotation in degrees, l is the path length in decimeters and c is the concentration in g/mL. Specific rotation is reported with the temperature and wavelength used, conventionally the sodium D line at 589 nm, as [α]_D²⁰.

Pure enantiomers of the same compound have specific rotations of equal magnitude and opposite sign. (S)-(+)-carvone is +61°; (R)-(−)-carvone is −61°.

Racemic mixtures and enantiomeric excess

A 50:50 mixture of two enantiomers is a racemic mixture or racemate, written (±). Its net optical rotation is zero, because the two contributions cancel exactly. Any reaction that makes a chiral product from achiral starting materials, using achiral reagents, gives a racemate — there is nothing in the system to prefer one mirror image over the other. SN1 reactions do this routinely, and it is one of the diagnostic features that distinguishes SN1 from SN2.

When a mixture is not 50:50, its purity is reported as enantiomeric excess:

An ee of 90% means 95% of one enantiomer and 5% of the other — the "excess" is the amount left over after the minor enantiomer has paired off with an equal amount of the major one. Asymmetric synthesis, one of the central goals of modern organic chemistry, is the art of running reactions with high ee.

Why they behave differently in biology

Enantiomers are indistinguishable in an achiral environment and completely distinguishable in a chiral one. An enzyme active site, a receptor, a transporter — all are built from chiral amino acids and are themselves chiral. Only one enantiomer may fit, in the same way that only one of your hands fits a right-handed glove.

The mechanism is worth stating precisely. When a chiral receptor binds a chiral molecule, the two possible complexes are diastereomeric, not enantiomeric — and diastereomers have genuinely different energies. That is why a chiral environment can discriminate where an achiral one cannot, and it is the same principle that makes resolution possible.

Separating enantiomers

Because enantiomers share every ordinary physical property, none of the standard purification methods work: distillation, recrystallization and normal chromatography cannot tell them apart. Separating them is called resolution, and it requires introducing chirality from outside.

The classical method reacts the racemate with a single enantiomer of a chiral resolving agent — a natural product such as tartaric acid or brucine — converting the pair of enantiomers into a pair of diastereomeric salts. Those have different solubilities, so they can be separated by ordinary crystallization, and the resolving agent is then removed. Modern practice more often uses chiral chromatography, where the stationary phase is chiral and the two enantiomers travel at different rates. In both cases the principle is identical: use a chiral environment to turn an enantiomeric relationship into a diastereomeric one.

What carries forward

Enantiomeric relationships are half of the stereoisomer taxonomy; the next section supplies the other half. Racemization is a diagnostic outcome in Module 6, telling you a reaction went through a planar carbocation. And the idea that a chiral environment converts enantiomers into diastereomers is the basis of every asymmetric reaction and every chiral separation in the laboratory.